Structural diversity of photoswitchable sphingolipids for optodynamic control of lipid microdomains.

Sphingolipids are a structurally diverse class of lipids predominantly found in the plasma membrane of eukaryotic cells. These lipids can laterally segregate with other rigid lipids and cholesterol into liquid-ordered domains that act as organizing centers within biomembranes. Owing the vital role of sphingolipids for lipid segregation, controlling their lateral organization is of utmost significance. Hence, we made use of the light-induced trans-cis isomerization of azobenzene-modified acyl chains to develop a set of photoswitchable sphingolipids with different headgroups (hydroxyl, galactosyl, phosphocholine) and backbones (sphingosine, phytosphingosine, tetrahydropyran-blocked sphingosine) that are able to shuttle between liquid-ordered and liquid-disordered regions of model membranes upon irradiation with UV-A (λ = 365 nm) and blue (λ = 470 nm) light, respectively. Using combined high-speed atomic force microscopy, fluorescence microscopy, and force spectroscopy, we investigated how these active sphingolipids laterally remodel supported bilayers upon photoisomerization, notably in terms of domain area changes, height mismatch, line tension, and membrane piercing. Hereby, we show that the sphingosine-based (Azo-ß-Gal-Cer, Azo-SM, Azo-Cer) and phytosphingosine-based (Azo-α-Gal-PhCer, Azo-PhCer) photoswitchable lipids promote a reduction in liquid-ordered microdomain area when in the UV-adapted cis-isoform. In contrast, azo-sphingolipids having tetrahydropyran groups that block H-bonding at the sphingosine backbone (lipids named Azo-THP-SM, Azo-THP-Cer) induce an increase in the liquid-ordered domain area when in cis, accompanied by a major rise in height mismatch and line tension. These changes were fully reversible upon blue light-triggered isomerization of the various lipids back to trans, pinpointing the role of interfacial interactions for the formation of stable liquid-ordered domains.

Optical manipulation of sphingolipid biosynthesis using photoswitchable ceramides.

Ceramides are central intermediates of sphingolipid metabolism that also function as potent messengers in stress signaling and apoptosis. Progress in understanding how ceramides execute their biological roles is hampered by a lack of methods to manipulate their cellular levels and metabolic fate with appropriate spatiotemporal precision. Here, we report on clickable, azobenzene-containing ceramides, caCers, as photoswitchable metabolic substrates to exert optical control over sphingolipid production in cells. Combining atomic force microscopy on model bilayers with metabolic tracing studies in cells, we demonstrate that light-induced alterations in the lateral packing of caCers lead to marked differences in their metabolic conversion by sphingomyelin synthase and glucosylceramide synthase. These changes in metabolic rates are instant and reversible over several cycles of photoswitching. Our findings disclose new opportunities to probe the causal roles of ceramides and their metabolic derivatives in a wide array of sphingolipid-dependent cellular processes with the spatiotemporal precision of light.

[5]Radialene.

The [n]radialenes are a unique family of fundamental [n]-membered carbocyclic structures with radiating alkenes, which have attracted significant synthetic and theoretical attention. Whereas [3]-, [4]-, and [6]radialenes have been prepared and studied, all efforts to synthesize the five-membered ring compound have thus far met with failure. Here we describe the first synthesis of the fundamental hydrocarbon [5]radialene, C10H10. Our approach was a departure from previous radialene syntheses in that it utilized a low-temperature decomplexation of a stable organometallic compound, rather than high-temperature elimination or rearrangement. Our strategy was guided by analysis of previous radialene syntheses, which indicated rapid decomposition in oxygen, and ab initio calculations, which revealed an extraordinary susceptibility of [5]radialene to undergo Diels-Alder dimerization/polymerization. The origin of this susceptibility was traced to a small distortion energy associated with the formation of the transition structure geometry from the relaxed reactant monomers and to a narrow HOMO-LUMO gap.

On the Diels-Alder dimerisation of cross-conjugated trienes.

The first general synthesis of 1-substituted [3]dendralenes has led to the discovery that conjugating groups significantly enhance the rate of Diels-Alder dimerisation relative to both the parent [3]dendralene and to other substituted systems.

Synthesis and applications of tricarbonyliron complexes of dendralenes.

[3]Dendralene and [4]dendralene are converted smoothly into tricarbonyliron complexes. The structures of four complexes analyzed by DFT and single-crystal X-ray analysis show that, in contrast to free hydrocarbons, complexed dendralenes prefer a roughly in-plane conformation. The complexes are stable towards Fe(CO)(3) group migration up to 150 °C. The synthetic value of Fe(CO)(3) complexation in the dendralene series is demonstrated through a variety of selective synthetic manipulations (Diels-Alder reaction, dipolar cycloaddition, Simmons-Smith cyclopropanation, dihydroxylation, olefin cross metathesis) that are not achievable by direct transformation of the free hydrocarbons. Application to the synthesis of a previously unreported, highly reactive linear/cross-conjugated hydrocarbon is also described.